We present the realization of a traveling-wave electrical pulse generator using pure nonresonant instantaneous optical rectification in bulk GaAs. The optical excitation was achieved by far-infrared pulses of 1–6 ps duration in the wavelength range from 8 to 15 μm, generated by a free-electron laser. The coupling of the optical rectificationpolarization into the fundamental mode of the microstrip transmission line is verified by angle-resolved measurements. Since optical femtosecond pulses are now becoming readily available, this alternative technique, which gains in efficiency at shorter pulses, may find growing importance for ultrafast pulse generation.

We demonstrate a method for efficient photon harvesting in organic thin films, thereby increasing the efficiency of organic photovoltaiccells. By incorporating an exciton-blocking layer (EBL) inserted between the photoactive organic layers and the metal cathode, we achieved an external power conversion efficiency of 2.4%±0.3% in vacuum-deposited ultrathin organic bilayer photovoltaic(PV)cells employed in a simple light trapping geometry. Ultrathin (∼100 Å) cells incorporating the transparent, conductive EBL have an internalquantum efficiency as high as 33%±4% over a spectral region matched to the solar spectrum. The very thin organic layers have a low series resistance, allowing for efficient power conversion in organic PVcells under intense (>15 suns) AM1.5 illumination. This device structure demonstrates that control of exciton diffusion in solid-state organic devices leads to a significant increase in the photon-to-carrier conversion efficiency.

The threshold current and the external efficiency of a three-section distributed Bragg reflector laser are investigated as a function of a forward electrical bias applied to the passive sections. Within a well-determined range, the threshold current increases and the external efficiency decreases with increasing bias. This effect is attributed to band gaprenormalization.

We developed a promising method to fabricate three-dimensional microstructures by using single-photon-absorbed polymerization confined to the vicinity of a tightly focused spot. This localized polymerization is based on the nonlinear response of the photopolymerizable resin to optical intensity with sufficiently low exposure. The nonlinear response was verified by measuring polymerization exotherm at different light intensities. The proposed method enables us to make even movable microstructures without any of the supporting parts or sacrificial layers normally required with conventional micromachining. In the experiment reported here, we fabricated a microgear with an external diameter of 47 μm and an attached shaft.

In this letter, we report on the temperature dependence of the intrinsic recombination coefficients in long-wavelength quantum-well lasers. Unlike previous studies, we obtain the intrinsic recombination coefficients from carrier lifetime measurements with a correction for the carrier population in the barrier and separate confinement heterostructure region. Our results show that this carrier population not only affects the value of the recombination coefficients obtained but also their temperature dependence. We measure a significant increase in the intrinsic Auger coefficient with temperature indicating that the frequently reported temperature insensitivity of this coefficient is likely due to carriers spilling out of the wells at elevated temperatures and not an intrinsic property of the Auger process.

We investigated ZnCdSe/ZnSe quantum-dot structures which include planar and coherently strained three-dimensional islands with different sizes. Optical excitation of these islands well below the ZnSeband gap leads to a resonant enhancement of the longitudinal-optical (LO) phonon-scattering efficiency and makes the 2LO and 3LO multiphonon emission observable. Resonant excitation with a power density of about 1.3 MW/cm2 using a micro-Raman setup results in an exponential decrease of the 1LO, 2LO, and 3LO intensity with irradiation time. This decay behavior is not observed for pure ZnSe crystals and can be avoided for the ZnCdSe/ZnSe structures using much lower excitation densities. The decrease in intensity is accompanied by a shift of the LO mode to higher frequencies resulting from a lower cadmium concentration in the alloy. From these experimental findings, we conclude that resonant excitation at a certain power density leads to cadmium out-diffusion from the planar quantum dots, which shifts the resonance away from the excitation energy.

We report in this letter on the strong influence of the polarization state of the probe beam in the amplitude and phase of the collinear mirage deflection. A model of the collinear mirage deflection that takes into account the photoelastic effect in the sample has been developed. The agreement between the model predictions and the experimental results is excellent.

Three-dimensional (3D) photonic crystalstructures can be fabricated into photopolymerizable resins by using laser beam interference with high precision. Three laser beams interfere into a glass cell filled with a liquid photopolymerizable resin to form a hexagonal periodic structure. Rods are formed in a hexagonal arrangement after being photopolymerzed according to the 3D periodic light distribution which results from the laser’s interference. Two beams of another laser also interfere to form layers which cross perpendicular to the rod array. After photofabrication, the nonsolidified resin is removed by ethanol. The lattice constant can be selected by tuning the angles of the incident beams and the laser wavelength. We have fabricated a photonic crystalstructure, the lattice constant of which is 1 μm and contains 150 lateral layers.

A two-beam polarizationinterferometer in a reflection configuration is used to measure the electro-optic coefficients of highly oriented strontiumbariumniobate thin films prepared by a sol-gel method. The technique enables the determination of the electro-optic coefficients of films using a strong Fabry–Perot effect with automatic adjustment and maintenance of the operation point of the interferometer. The linear electro-optic coefficients increase with increasing Sr content in the films.

The interface of direct bonded GaAs to GaAs has been studied by scanning transmission electron microscopy and electron energy loss spectroscopy. Voids are seen along the boundary with most being partially filled with a gallium particle. Two general sizes of voids are seen. The large voids are distributed in an approximately linear relationship and the smaller randomly. In compliant substrates, one of the layers is made thin and twisted The larger voids often extend past this thin compliant layer, but no evidence of granularity of the epitaxial film is observed.

Epitaxial Si/InAs/Si heterostructuregrown on (001) Si substrate by molecular beam epitaxy and annealed at was investigated by high resolution transmission electron microscopy. Extensive interdiffusion leads to the formation of an InAs solid solution in the Si cap layer. Additionally, InAs-enriched regions with extensions of which exhibit two kinds of ordering are observed. The ordering of InAs molecules has occurred, respectively, in (101) and planes inclined and (110) and planes parallel to the [001] growth direction. It is attributed to the energy gain from the reduced number of mixed Si–As and Si–In bonds. The sample show photoluminescence in the 1.3 μm region, which is tentatively attributed to the recombination of excitons localized in the ordered regions.

We report on the growth and characterization of high-quality strain-relaxed SiGe alloys on a compliant silicon–on–insulator (SOI) substrate. The annealing temperature required for strain transfer has been reduced through boron implantation to the buried oxide, leading to a high quality SiGe alloy free from dislocations as evident from the near-band gap photoluminescence. Nearly complete strain relaxation (∼95%) for SiGe alloy of a thickness beyond the conventional critical thickness has been obtained.

Excited state radiative lifetime measurements are made on porous silicon as a function of excitation wavelength and excitation intensity. The results indicate that a simple quantum confinement model for the light absorption and emission mechanism is not suitable. We support our results by suggesting that a cascading energy transfer process among surface molecule-like states is most likely active and we provide a general indication of the density of energy transfer states.

We report the preparation of periodic nanostripes on vicinal copper surfaces. For the investigated surfaces an oxygen-induced mesoscopic faceting of the regular monoatomic stepped surfaces into periodic nanostripes consisting of Cu(111) and facets is observed. The width and thermal stability of these nanostripes increase with the terrace length of the initial vicinal surfaces. Stripe widths of 50, 20, and 12 nm were obtained for Cu(443), Cu(332), and Cu(221), respectively. Whereas on Cu(221) the nanostripes disappear above 450 K, they are stable up to 800 K on Cu(443). For the latter surface, the nanostructures are found to be unusually stable and could be observed ex situ by atomic force microscopy under ambient conditions.

We propose a mechanism to explain the anomalous degradation of silicon space solar cells. Distinct from previously known mechanisms, it has been shown that the anomalous increase and abrupt decrease of short-circuit current are caused by corresponding changes of the minority carrier lifetime and a conversion of conductivity type. The majority carrier density decreases abruptly due to trapping by the radiation-induced deep donors, which results in an increase of carrier lifetime and resistance, conversion of conductivity type, and anomalous change of solar cell performance. Peak values of the carrier lifetime and short-circuit current decrease with increasing illumination intensity and are sensitive to variations of the weak optical illumination.

We demonstrate that femtosecond pump–probe spectroscopy in the optical near field is well suited to study the intrinsic properties of single V-groove GaAs quantum wires. Temporally and spatially resolved experiments show that the shape of near-field pump–probe traces sensitively depends on the detuning between the laser photon energy and the lowest exciton resonance of a quantum wire. This detuning dependence allows one to map the quantization energy fluctuations along a single quantum wire with 200 nm spatial resolution. We measure fluctuations of about 12 meV over 2 μm wire length, resulting from wire thickness variations of 1 ML.

We have studied the early stages of annealing in boronimplantedsilicon. In a grazing incidence diffuse scattering investigation of implantation-induced defects, we have observed narrow diffuse rods of intensity along 〈111〉 directions. These diffuse streaks arise from stacking faults formed during annealing in the 1000 °C range. From the width of the diffuse streak the average size of the stacking fault is 71 nm in diameter. These intensity rods are distinct from the point defect or point defectclusterscattering in the tails of the Bragg peak (Huang scattering). From the q dependence of the scattered intensity in the Huang scattering region we find clear evidence for defect clusters with an average effective size of 4 nm, remarkably independent of the annealing temperature. These observations are discussed in the context of the enhanced diffusion of implantedboron over its bulk value referred to as transient enhanced diffusion.

A GaN pyramid grown selectively on a (111)Si substrate with a patterned dot structure of a mask, by metalorganic vapor phase epitaxy using AlGaN as an intermediate layer, was characterized by transmission electron microscopy. The dot pattern has an array of 5.0-μm-diameter window openings with a 10 μm period. The density of threading dislocations observed in the window region decreased gradually with increasing distance from the interface. This was mainly due to the dislocation reaction and bending of threading dislocations for the first 2 μm region from the interface and for the upper region, respectively. Dominantly observed defects in the lateral-growth part were dislocations parallel to the interface. An amorphous layer was formed at the interface in the window region. Nitride particles were observed at the interface in the mask region.

Ultraviolet photoelectron spectroscopy has been applied to the investigation of modified hole injection barriers in organic light-emitting devices(OLEDs). Different from those reported previously, the indiumtin oxide (ITO) surfacetreatedin situ by oxygen plasma possesses a work function of 5.2 eV, and the organic ITO interface thereafter formed shows a 0.5 eV smaller hole injection barrier compared to that on untreated ITO. Insertion of an ultrathin layer between the organic and ITO results in a similar reduction of the barrier. This indicates that improved hole injection favors efficient operation of OLEDs, as manifested by enhanced efficiency by the insertion.